The present invention relates generally to an improved system and method of vehicle control. More specifically, the present invention is directed to a Communications based Train Control (CBTC) system that utilizes low-cost, readily available hardware to control and direct various trains in a safe and efficient manner.
For over a hundred years the movement of trains, or other track guided vehicles, has been controlled such that increasing numbers of vehicles can operate within a network of tracks in a safe and efficient manner. Both people and freight are transferred on trains between locations separated by distances ranging from hundreds of feet to thousands of miles. With a single train running on a single track or network of tracks, with no obstacles, control of the train is simple. Since there is little concern for the train coming into contact with any other objects, the train can run at maximum speed, limited only by the speed performance of the train, the train""s stopping ability once it reaches its destination, and the train""s ability to stay on the track, i.e., while travelling around turns, etc.
However, as additional trains are placed on the track, or track network, to take advantage of the unused capacity of the tracks and provide viable transportation alternatives, controlling the trains to keep them operating in a safe and efficient manner becomes more complex. For example, as two trains approach one another from opposite directions, in order to avoid a collision, one of the trains must be switched to another track. Similarly, as two trains approach one another from the same direction, i.e., on the same track with the one behind the other travelling at a faster speed than the train in front, either the train in front must be sped up, or the one behind must be slowed down. Accordingly, Railways are provided with signaling primarily to ensure that there is always enough space between trains to allow one to stop before it hits the one in front.
In typical systems, signaling is achieved by dividing each track into sections or xe2x80x9cblocksxe2x80x9d, which is a length of track of defined limits. The length of a block is usually determined to be the distance it takes a train, running at full speed, to come to a complete stop under the worst possible conditions. Each block is protected by a signal placed at its entrance. If the block is occupied by a train, the signal will display a red xe2x80x9caspectxe2x80x9d, to instruct the conductor to stop the train. If the section is clear, the signal can show a green or xe2x80x9cproceedxe2x80x9d aspect.
A track circuit is typically the mechanism by which the presence of a train in a block is usually detected. Many rail-lines with moderate or heavy traffic are equipped with color light signals operated automatically or semi-automatically by track circuits. When the track circuits detect a train, the signal shows a red aspect. If no train is detected and the circuit is complete and the signal shows a green aspect (or yellow, in a multi-aspect signaled area).
A low voltage from a battery is applied to one of the running rails in the block and returned via the other rail. A relay at the entrance to the section detects the voltage and is energized to connect a separate supply to the green lamp of the signal.
When a train enters the block, the leading wheelset short circuits the current, which causes the relay to de-energize and drop the contact so that the signal lamp supply circuit activates a red signal lamp. The system is xe2x80x9cfail-safexe2x80x9d, or xe2x80x9cvitalxe2x80x9d as it is sometimes called, when any break in the circuit will cause a danger signal to be displayed.
The above is a simplified description of a track circuit. Actually, a xe2x80x9cfixed-blockxe2x80x9d section is conventionally electrically separated from its adjacent sections by insulated joints in the rails. However, more recent installations use electronics to allow jointless track circuits. Also, some areas have additional circuits which allow the signals to be manually held at xe2x80x9credxe2x80x9d from a signal box or control center, even if the section is clear. These are known as semi-automatic signals.
The development of signaling and train control technology can generally be separated into two periods, with 1920 as the dividing point. Before 1920, the major areas of technological advance were interlocking control and block signaling (manual and automatic).
After 1920, the demand for moving heavier traffic at higher speeds and with increased safety led to major developments such as centralized traffic control, continuous cab signaling, coded track circuits, and automatic train control (ATC). Generally, innovative signaling and train control technology for rail rapid transit was derived from railroads and lagged behind railroad application by about 10 years. There were some notable exceptions; the development of automatic junction operation and automatic train dispatching was pioneered in rail rapid transit. Very recently, since roughly 1960, there has been some experimentation with techniques and equipment solely for rail rapid transit and small people-mover systems.
Over the years, technological advances in several areas of communication has lead to vast improvements in train control. For example, centralized control has typically replaced the need for block signaling such as described above.
The original and most important purpose for control devices and/or systems is to prevent collisions between vehicles moving in the track network. For this purpose, as mentioned above, it has been known to divide the line into blocks and to prevent, by central control, any train from entering into a block unless the block is free of other vehicles. This xe2x80x9creal-blockxe2x80x9d type of system may be suitable for less dense traffic, however, it is not suitable for use within track networks where the traffic has to be dense and where the length of the blocks would, thus, have to be extremely short, leading to major investment and control cost.
One known conventional system provides a calculation of the movement within a block by means of a message sent to a central unit from the train about its speed. The central unit then performs a distance calculation by multiplying the train""s speed by the desired time increment. Accordingly, the speed may be centrally controlled if a collision risk occurs. By being able to determine, at least approximately, the position within a block of each train, several trains can be permitted into the same block as long as the central surveillance unit, as well as the communication with the train, functions properly. By using this method of calculating train positions, however, the position determinations obtained are so uncertain, that either the blocks must be made very small, so that the calculation must be updated frequently, or the number of allowed trains within the same block must be strictly limited. Also, as the demand to increase traffic density rises, prohibitively small blocks would be required, making it practically impossible to build such a system at a reasonable cost and with a reasonable control capacity.
Another known conventional train control system also includes dividing the tracks into blocks where, within each block, movement of the train is determined by means of a rotation meter on the wheels of each train. The position determination within the block is then made centrally by emitting clock pulses that are returned by the train with a delay corresponding to the distance of travel within the block, measured by the rotation meter.
In both of the conventional systems mentioned above, the passage of each train past a block borderline is reported to the central unit, whereupon information about speed and distance traveled is repeatedly determined. The central unit calculates the location of each train within the block and controls the velocity of at least one of the trains to avoid a collision, if two or more, trains are approaching each other.
The conventional systems, thus described, require a physical division of the track network into blocks, with installations that, when passed by a mobile unit on each train, trigger the central unit calculation of the distance traveled by means of a repeated exchange of information between the central unit and the mobile units. This results in a requirement for very frequent communication between the central unit and each mobile unit. Should this communication break down, for any reason during a period of time, the security of the position determination is lost. This might indicate that cable-based signal transmission should be chosen for safety reasons. As the methods used for calculation of the distance traveled will necessarily produce a result having considerable tolerances, the blocks must have a limited length unless the safety distances between the trains can be made very long.
The mentioned systems are primarily applicable to train traffic over longer distances on railway lines, as their traffic generally is not so frequent and the safety distances can be made long. This makes a division of the railway line into blocks of considerable length, and thus of limited number, possible. For urban tramways, however, the conditions are considerably more complicated as dense traffic, as well as strongly varying speeds, is necessary. Under these conditions, the blocks would have to be very short in order for the tolerances of the calculated distance traveled within the block to not risk the safety of the position determination.
A communications-based train control system has been suggested wherein the concept of dividing the track network into blocks is eliminated and there is thus no indication to a central unit of the passing of each train past block borderlines. Instead, the position within the track network of each train is calculated on-board each train by distance measurements taken during travel. In order for the position determination to be held within close enough tolerances such that dense traffic can be permitted without safety risks, a calibration of the position determination process is performed over a series of short intervals by passive elements at determined fixed points, by means of transponders scanned by radio equipment on board the train. The determined position of the train is then transmitted by wireless communication to a central unit, which may thereby calculate the distance between different trains, for speed control and for any possible emergency braking.
Specifically, in accordance with the above-mentioned system, the mobile unit on-board the train includes distance meters. The distance meters further include pulse counters mounted on the wheel axles and are used for measuring the distance traveled during a particular time interval. In this way, the position and the speed of the train can be determined. In practice, at least two measuring wheels are necessary in order to detect slippage, blockage and any possible pulse counter function errors.
A distance meter, however, will unavoidably lead to an accumulated error in the distance measurement. For example, wheels of a train have a tendency to xe2x80x9cslip,xe2x80x9d xe2x80x9cslidexe2x80x9d and xe2x80x9cspinxe2x80x9d referring to various situations where the rotational speed of the wheels does not correspond with the actual rolling contact between the wheel tread and the rail surface. Accordingly, redundant counters are often used and calibration of the measured distance must be performed often. In the system mentioned here, calibration takes place every time the train passes a fixed number of points in the track network and is preferably performed at every stop. Calibrating the distance measurement is done by a radio frequency sensor on-board the train that registers the passage of a passive transponder placed in the ground between the tracks or suspended from the current supply line.
In addition, U.S. Pat. No. 4,735,383 describes a railway control system in which a plurality of transponders are positioned at intervals spaced along a track. Each passing train within the system has radio equipment for reading the identity of a passed transponder. Each train then transmits the transponder identity and information about its own identity to a central station. The central station then provides each train with signaling information. The central station, however, provides signaling information to only one train at a time using a single radio channel. Because individual messages are sent serially to each of the trains, this system requires the central station to provide very short broadcasts to each train.
U.S. Pat. No. 5,740,046 describes a method for controlling vehicles in a tram line which uses a number of passive beacon tags to determine a tram""s position. In this system, the length of the tracks is divided into separate cells. A central system communicates with the trams by sending messages, each of which is intended for an individual tram. In order to only reach an individual tram, each messages is transmitted only within the individual cell in which the intended tram is located. To reach all of the trams within a track area would therefore require multiple transmissions from the central system.
Thus, it would be advantageous to use a vehicle control system that provides a simple transmission of detailed information to all of the trains within a track area.
The present invention is directed to a vehicle control system and method in which a plurality of beacon tags are disposed along a length of a track for a predetermine number of blocks. The beacon tags each provide identification information pertaining to the tag""s location.
Each vehicles that passes along the track has a tag reader that solicits information from the beacon tags and a transmitter that transmits the solicited information, as well as vehicle identification information for the transmitting vehicle, to a wayside control unit. The wayside control unit receives the transmitted position information and vehicle identification information and in turn transmits a single broadcast of information pertaining to each of the blocks of the predetermined number of blocks. This signal is received by all of the vehicles, which use only the information about immediately approaching blocks.
In addition, dynamic tags located at positions along the length of the predetermine number of blocks can be used as a backup system for providing the same information that is provided by the wayside control unit.